Have you tried to plug in some values for x to find the y values? That would be a useful way to see the pattern, If you can start with x=0 and continue with x=1,2,3,... you can see the behaviour of this equation, which is an exponential decay. Plugging in values for variables is a useful way to understand the behaviour of the function. You can double check this with using a graphing calculator. Secondly taking a number between 0 and 1 to a power results in a smaller number. Therefore for the larger x values, y will get smaller.
How do you determine if the equation #y = 2.1(0.15)^x# represents exponential growth or decay?